1. Field of the Invention
The invention pertains generally to the fabrication of devices and, more particularly, to the fabrication of semiconductor devices.
2. Art Background
The fabrication of devices, e.g., semiconductor devices, typically involves the etching of patterns into substrates, e.g., (1) silicon wafers or (2) silicon wafers which are being processed and are thus wholly or partially covered by regions of materials such as metal, polysilicon, or silicon dioxide. Generally, a pattern is etched into a substrate by initially forming a masking layer, e.g., a resist, on the substrate, removing selected portions of the masking layer, and then etching the exposed portions of the substrate.
It is possible to produce either isotropic etching or anisotropic etching in the exposed substrate portions. (In isotropic etching, a layer of material is etched in both a first direction, perpendicular to the major surface of the layer, and a second direction, transverse to the first direction, the etch rates in the two directions being approximately equal. In anisotropic etching, the etch rate in the second direction is less than that in the first direction.) If very fine line features are to be etched into the exposed portions of a layer of substrate material, then anisotropic etching is generally preferred over isotropic etching to avoid obliterating the features (through transverse etching) during the time required to etch the layer through its thickness. Thus, anisotropic etching is useful, among other things, for producing the very fine line etching required in the fabrication of a variety of devices including, for example, very large scale integrated (VLSI) semiconductor devices.
One method for producing anisotropic etching is reactive ion etching (RIE). In RIE, a substrate to be etched is mounted on an electrode arranged within a vacuum chamber, an etchant gas is introduced into the chamber, and a plasma (which includes electrons and positive and negative ions) is established within the gas. Typically, the substrate includes several different layers of material, only one of which is to be etched. Thus, the etchant gas is chosen so that the plasma produced in the gas includes chemically reactive positive or negative ions which selectively etch the substrate, i.e., selectively chemically etch the one layer of the substrate to be eched. The chemical etching results in the formation of a volatile reaction product which is pumped out of the chamber.
If the electrode on which the substrate is mounted is, for example, a free-floating electrode (an electrode whose electrical potential is allowed to float), then the electrode (by virtue of its partial or total immersion in the plasma) will develop a negative or positive DC bias relative to a point of reference potential, such as ground. (The DC bias is negative or positive depending on the nature of the plasma, the position of the electrode within the plasma, and the material properties of the electrode.) Under the influence of the DC bias, either positive or negative ions (depending on whether the DC bias is negative or positive) are accelerated toward the electrode. The directed motion imparted to the ions by the DC bias, as well as the chemical reactivity of the ions, results in the selective, anisotropic chemical etching of the substrate.
The magnitude of the DC bias at the electrode significantly affects, for example, the degree of the anisotropic etching (the smaller the magnitude of the DC bias, the lower the degree of anisotropic etching) and thus the shape of the etch profile. In addition, the magnitude of the DC bias significantly affects the etch selectivity between a substrate layer to be etched and material layers not to be etched, e.g., the masking material or other layers of substrate material. For example, a DC bias just sufficient to impose directionality on the reactive species will draw the reactive species to the electrode at a speed sufficient to cause sputter removal of a relatively small amount of masking material or substrate material which is not to be removed. However, an increase in the magnitude of the DC bias significantly above that required to impose directionality will result in the sputter removal of an undesirably large amount of material which is not to be removed. Consequently, control of DC bias is highly desirable, among other things, to achieve a desired etch profile, e.g., an essentially vertical, inclined, or curved etch profile, and to avoid undesirably large reductions in etch selectivity.
There is a variety of techniques for producing RIE. The differences between these techniques lie, in part, in the methods employed to produce the plasma. For example, in one technique the plasma is produced by a DC discharge, i.e., by imposing a sufficiently high DC voltage between two electrodes placed across (but not necessarily immersed within) a gas. (Regarding plasmas formed by DC discharges see, e.g., B. Chapman, Glow Discharge Processes (John Wiley & Son, 1980), p. 77.) Such a technique is adequate in many situations. However if either the substrate being etched, or the electrode supporting the substrate, includes dielectric material, then etching is difficult to maintain.
In another technique for producing RIE the plasma is formed by inductively coupling AC power into the gas. Such inductive coupling is achieved, for example, by applying an AC voltage across the primary winding of a transformer whose secondary winding encircles a chamber containing the gas. Alternatively, an AC voltage is directly applied to an induction coil which either encircles the chamber or is arranged within the chamber. (In regard to inductive coupling see, e.g., J. Vossen and W. Kern, eds., Thin Film Processes (Academic Press, 1978), Ch. 4, pp. 338-341.) Again, this technique is employed to advantage in many situations. However, in this technique the ions acquire a helical-type motion (in addition to the directed motion imparted by the DC bias on the substrate-supporting electrode) that reduces somewhat the degree of anisotropic etching.
The most widely used commercial technique for producing RIE, which employs radio frequency (rf) power for plasma production, does not have the limitations present in the other methods. An RIE machine which employs such rf power is the conventional, parallel plate RIE machine 10, depicted in FIG. 1, which includes two electrodes 18 and 20 arranged within a vacuum chamber 12. The RIE machine 10 also includes an inlet 16 through which an etchant gas (or gases) is introduced into the chamber 12, as well as an outlet 14 through which volatile reaction products are pumped out of the chamber 12.
RF power is capacitively coupled into an etchant gas introduced into the chamber 12 by connecting one of the electrodes, e.g., the electrode 18, to a point of reference potential, such as ground. In addition, an rf voltage is applied to the other electrode, e.g., the electrode 20, by an rf voltage generator 24, through an impedance matching network 26 and capacitor 28 (hence, capacitive coupling), as shown in FIG. 1. The electrode 18 in this particular configuration thus constitutes the anode while the electrode 20 constitutes the cathode of the RIE machine 10.
The application of the rf voltage signal, or any other form of power, not only produces a plasma in the gas, but also results in the cathode 20 acquiring a negative (relative to the grounded anode 18) DC bias. In operation, a substrate 22 to be etched is mounted on the cathode 20 (see FIG. 1) and undergoes anisotropic chemical etching by positive ions accelerated toward the cathode by its negative DC bias. (Mounting the substrate on the anode 18 generally leads to isotropic, rather than anisotropic, etching.)
It is generally believed that the DC bias at the cathode 20 is due to the formation of a sheath of electrons at the surface of the cathode 20. The capacitor 28, which presents a high impedance to low-frequency and DC signals (while presenting a low impedance to the high-frequency signal produced by the rf generator 24), blocks the electron sheath (a DC signal) from being discharged through the rf generator to ground.
The magnitude of the DC bias at the cathode 20 is largely determined by the amount of rf power delivered to the cathode 20, as well as the pressure and composition of the gaseous atmosphere within the chamber 12. (Power, pressure, and gas composition are also largely determinative of the DC bias in RIE systems which employ different power producing means, e.g., DC discharges or inductive coupling.) Because these factors are readily controlled, DC bias is readily controlled, thus permitting enhanced control of, for example, etch profile and etch selectively. For example, an increase in rf power (achieved by increasing the amplitude and/or frequency of the rf voltage signal) and/or a decrease in pressure results in a higher, i.e., a more negative, cathode DC bias.
The factors determining the magnitude of the DC bias, also determine the etch rate (irrespective of the nature of the power producing means). For example, an increase in rf power results in an increase in etch rate, as well as an increase in the magnitude of the DC bias. In fact, the magnitude of the DC bias is a reliable measure of rf power flow and etch rate. Indeed, it has long been believed that the DC bias is so intimately intertwined with the rf power flowing into the plasma that any attempt to decrease DC bias (independently of rf power and pressure) by, for example, withdrawing current from the cathode, will necessarily result in a significant decrease in rf power flowing to the plasma, and thus a significant decrease in etch rate.
A high etch rate is generally desirable for purposes of economy. But a high etch rate is often accompanied by an undesirably high DC bias, which results, for example, in undesirably low-etch selectivity. Moreover, if the substrate being etched includes one or more layers of dielectric material, then these layers often suffer undesirable electrical breakdown when exposed to a high DC bias.
The constraints imposed on the factors which determine etch rate and DC bias, e.g., rf power and pressure, conflict when both a high etch rate and, for example, a high etch selectivity are desired. Consequently, etch rates are generally maintained at a relatively low level to achieve high etch selectivities. Although a technique for controlling the DC bias, independently of rf power and pressure, to achieve both high etch rates and high etch selectivities has been sought, a solution has not be found.